Silicon carbide is a semiconductor material having excellent characteristics, such as a large band gap, high thermal conductivity, high electron saturation drift velocity and high dielectric breakdown voltage, and is drawing attention as a material for next-generation low-loss power device elements. As in the case of silicon semiconductors, thermal oxidation can be used to form an oxide film from silicon carbide, making a field-effect transistor having a metal-oxide film-semiconductor structure (MOSFET) a leading candidate for silicon carbide power device elements. But with silicon carbide substrate MOSFETs manufactured with current technology, channel mobility is far lower than the mobility predicted from the electron mobility of bulk silicon carbide. This is also the case with a MIS-type FET that uses an oxide film/nitride film/oxide film or the like as the insulation film. Considerable research has been conducted aimed at improving the channel mobility, and there are a number of proposals relating to a method of forming the oxide film/silicon carbide interface.
For example, in thermal oxidation of pyrogenic oxidation in which water is produced by the combustion of hydrogen and oxygen, JP-A HEI 11-31691 discloses a method in which the flow rate of hydrogen is increased to more than the flow ratio of 1:1 between hydrogen and oxygen. The embodiments thereof describe using this method to form a gate oxide film, decreasing the interface state density in a MOS capacitor using P-type silicon carbide.
With respect to the method of oxidizing a silicon carbide substrate, U.S. Pat. No. 5,972,801 discloses using the thermal oxidation method to form a gate oxide film in a range of 1050° C. to 1100° C., and following this by heat treatment at 600° C. to 1000° C. in an atmosphere containing H2O. In the embodiments thereof, oxygen is passed through pure water heated to 95° C. (subjected to bubbling) to produce an atmosphere containing H2O.
Because a conventional MIS- or MOS-type FET that uses a silicon carbide substrate has a channel mobility that is much lower than the electron mobility of bulk silicon carbide, the intrinsic properties of silicon carbide are not reflected in the device characteristics. That is, the FET On resistance (Ron) is much higher than the value that is theoretically predicted based on the property values of silicon carbide. Although silicon carbide having the crystalline structure known as 4H (4H-SiC) has a bulk electron mobility in the order of 900 cm2/Vs, the channel mobility of a MOSFET formed by the usual thermal oxidation method is a very low 5 to 10 cm2/Vs. To improve this, Reference 1 (K. Ueno et al., Mat. Sci. and Eng. B61-62 (1999) 472-474) relates to an improvement by using surface treatment prior to formation of the gate oxide film; Reference 2 (J. A. Cooper et al., Mat. Res. Soc. Proc. Vol. 572, pp 3-14) relates to an improvement based on the use of a low activation annealing temperature; and Reference 3 (G. Y. Chung et al., IEEE Electron Device utt. 22, 176 (2001)) relates to re-oxidation treatment in an NO (nitrous oxide) atmosphere. These references report channel mobilities in the order of 20 to 35 cm2/Vs, so in view of the bulk electron mobility, dearly, there still is room for improvement.
The present invention was proposed to resolve the above problem, and has as its object to provide a method of manufacturing MIS- and MOS-type semiconductor devices with high channel mobility that use a silicon carbide substrate.